1. Technical Field
The present disclosure relates to a radar device.
2. Description of the Related Art
In recent years, studies have been conducted on a high-resolution radar device using a radar transmission signal having a short wavelength including a microwave or a millimeter wave. Furthermore, in order to improve outdoor safety, there are demands for development of a radar device (wide-angle radar device) that detects objects (hereinafter referred to as targets) including not only a vehicle but also a pedestrian in a wide-angle range.
For example, as a radar device, a pulse radar device that repeatedly emits a pulse wave is known. A wide-angle pulse radar that detects a vehicle and a pedestrian in a wide-angle range receives a signal that is a mixture of a plurality of reflected waves from a target existing at a short distance (e.g., a vehicle) and a target existing at a long distance (e.g., a pedestrian). Accordingly, (1) a radar transmitting unit need be configured to transmit a pulse wave or a pulse-modulated wave having an autocorrelation characteristic of a low range sidelobe (hereinafter referred to as a low-range-sidelobe characteristic) and (2) a radar receiving unit need be configured to have a wide reception dynamic range.
A pulse-compression radar device using a Barker code, an M sequence code, or a complementary code has been proposed as a radar device using a pulse wave or a pulse modulated wave for obtaining a low-range-sidelobe characteristic. For example, a complementary code is made up of two code sequences (complementary code sequences). For example, in a case where the two code sequences are an and bn (n=1, . . . , L (a code sequence length)), a result of adding (see the following formula (3)) results of autocorrelation computation of the two code sequences (see the following formulas (1) and (2) where an=0 and bn=0 in a case where n>L or N<1, and the asterisk (*) represents a complex conjugate operator) by uniforming respective shift times τ is 0 when τ≠0, and a correlation value in which a range sidelobe is 0 is obtained:
                                          R            aa                    ⁡                      (            τ            )                          =                              ∑                          n              =              1                        L                    ⁢                                          ⁢                                    a              n                        ⁢                          a                              n                +                τ                            *                                                          formula        ⁢                                  ⁢                  (          1          )                                                              R            bb                    ⁡                      (            τ            )                          =                              ∑                          n              =              1                        L                    ⁢                                          ⁢                                    b              n                        ⁢                          b                              n                +                τ                            *                                                          formula        ⁢                                  ⁢                  (          2          )                                        {                                                                                                                                                R                        aa                                            ⁡                                              (                        τ                        )                                                              +                                                                  R                        bb                                            ⁡                                              (                        τ                        )                                                                              ≠                  0                                ,                                                                                      when                  ⁢                                                                          ⁢                  τ                                =                0                                                                                                                                                                            R                        aa                                            ⁡                                              (                        τ                        )                                                              +                                                                  R                        bb                                            ⁡                                              (                        τ                        )                                                                              =                  0                                ,                                                                                      when                  ⁢                                                                          ⁢                  τ                                ≠                0                                                                        formula        ⁢                                  ⁢                  (          3          )                    
A method for generating a complementary code is disclosed in Budisin, S.Z., “New complementary pairs of sequences,” Electronics Letters, Vol. 26, Issue: 13, pp. 881-883, 1990. According to this method, complementary codes having code lengths L of 4, 8, 16, 32, . . . , and 2P can be sequentially generated on the basis of code sequences a=[1, 1] and b=[1, −1] that are complementary to each other in which an element is “1” or “−1”. A required reception dynamic range of a radar device is wider as the code length is longer. However, use of a complementary code allows a peak sidelobe ratio (PSR) to be made lower even in a case where the code length is shorter. Accordingly, even in a case where a plurality of reflected waves from a target existing at a short distance and a target existing at a long distance are mixed, the dynamic range of a radar device required for reception can be reduced. Meanwhile, in a case where an M sequence code is used, the PSR is given by 20 log(1/L), and a code length L (for example, L=1024 in a case where PSR=60 dB) that is longer than that in the case where a complementary code is used is needed in order to obtain a low range sidelobe.
Examples of a configuration of a wide-angle radar device include the following two configurations.
A first wide-angle radar device is configured to transmit a radar wave by mechanically or electronically scanning a pulse wave or a modulated wave by using a directional beam of a narrow angle (a beam width of approximately several degrees) and receive a reflected wave by using a directional beam of a narrow angle. In the wide-angle radar device of the first configuration, scanning need be performed many times in order to obtain high resolution, and therefore trackability of a target that moves at a high speed deteriorates.
A second wide-angle radar device is configured to use a method (Direction of Arrival (DOA) estimation) in which a reflected wave is received by an array antenna made up of a plurality of antennas (antenna elements) and an arrival angle of the reflected wave is estimated by a signal processing algorithm based on a reception phase difference corresponding to an array antenna spacing. In the wide-angle radar device of the second configuration, an arrival direction can be estimated on a reception side even in a case where a scanning interval of a transmission beam on a transmission side is shortened. It is therefore possible to shorten a scanning time and to improve trackability as compared with the wide-angle radar device of the first configuration. Examples of an arrival direction estimation method include Fourier transform based on matrix operation, a Capon method and an LP (Linear Prediction) method based on inverse matrix operation, and an MUSIC (Multiple Signal Classification) and an ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) based on eigenvalue computation.
Furthermore, a configuration (hereinafter referred to as an MIMO radar) in which a plurality of antennas (an array antenna) are provided not only on a reception side but also on a transmission side and beam scanning is performed by signal processing using the transmission and reception array antennas has been proposed as a radar device (see, for example, Jian Li, Petre Stoica, “MIMO Radar with Collocated Antennas”, Signal Processing Magazine, IEEE Vol. 24, Issue: 5, pp. 106-114, 2007).
In the MIMO radar, a virtual reception array antenna made up of as many antenna elements as the product of the number of transmission antenna elements and the number of reception antenna elements at most can be realized by appropriately disposing the antenna elements in the transmission and reception array antennas. This produces an effect of increasing an effective aperture length of the array antennas with a small number of elements.
Furthermore, a method for detecting the present or absence of a target in a wide-angle range by using an MIMO radar has been proposed. The MIMO radar transmits, via a plurality of transmission antennas, orthogonal multiplexed signals that can be separated on a reception side. For example, orthogonal code sequences (see, for example, C.C. Tseng, C. L. Liu, “Complementary sets of sequences”, Information Theory, IEEE Transactions on Vol. 18, Issue: 5, pp. 644-652, 1972) is applied as the orthogonal multiplexed signals.
Furthermore, Japanese Unexamined Patent Application Publication No. 61-96482 discloses a radar system that suppresses interference between sector radars by using mathematically orthogonal complementary codes as transmission codes in a plurality of (e.g., two) radars.
However, in the MIMO radar device using code multiplexing, as a relative speed between the MIMO radar device and a target increases, a Doppler phase fluctuation caused by Doppler frequency shift increases, and interference between code multiplexed signals increases. When interference between the code multiplexed signals increases, it becomes difficult to independently extract waves reflected by a target from respective antennas, positioning performance of the MIMO radar device deteriorates, and incorrect detection or failure of detection of a target are more likely to occur.